1. Field of the Invention
The present invention relates generally to the fabrication of nano-devices and semiconductor devices. More specifically, the present invention relates to direct write fabrication and lithography techniques and systems employing the Lorenz force interactions of light fields with the dipole moment of atoms to build stable nanostructures of matter via the direct deposition of atoms upon substrates.
2. Description of the Prior Art
The nano-device and semiconductor device manufacturing communities consistently strive to develop fabrication techniques and equipment that enable the precise definition of ever smaller structural components of functional devices. These device manufacturers expend extensive financial and technological resources in the exploration and development of improved photolithography process tools and methods on a continual basis.
The potential rewards for achieving significant reduction in minimal dimension manufacturable structures, features or spot sizes, are tremendous. Smaller structural components can be used to create smaller devices. Semiconductor devices constructed with smaller feature and structure dimensions operate at faster speeds, consume less power and embody higher functional complexities. Smaller nano-devices function within finer scales of precision. The economic motivation of manufacturers to improve over the capabilities of conventional photolithography has therefore led to the funding of much intensive work in the field of x-ray level photolithography.
Yet the fundamentals of prior art photolithography are self-limiting to enhancements in several ways. Conventional photolithography is a multi-step process. Each step typically contributes an error factor to a finished structure. An example of a photolithography process might include a deposition of a specific material onto a substrate, the application of a layer of photoresist over the substrate, a soft bake of the photoresist, an exposure of the photoresist layer to a pattern-masked light source, the development of the photoresist, a hard bake of the photoresist and finally an etching step.
Each step in this example photolithography process has limitations in accuracy and result. The validity of the deposition action is dependent upon the uniformity of the layer of material actually deposited. The resolution of the component structures of the device defined in the photoresist development steps is limited by the wavelength of light used, the geometric accuracy of the pattern-mask, the effects of unintended under or over exposure to the light source, and the equally unintended effects of under development or over development of the photoresist after exposure to the light source. The soft baking and/or the hard baking of the photoresist can also be over or under done. The precision of the etch step is affected by the degree of selectivity of the etching agent and the degree of anisotropy achieved in the delivery of the wet or dry etching agent across the entire surface of the substrate.
The errors introduced by each step of the photolithography process are further compounded by the fact that the fabrication of most semiconductor devices requires the repeated application of entire cycles of photolithography processes which employ widely varying deposition materials, photoresist compounds, etching chemicals, pressure and temperature requirements. The net result is often an increasing limitation in the smallest achievable individual structure size, as well as reduction in the control of precision in device feature fabrication.
The sheer number of individual process steps of a typical device fabricated with conventional photolithography techniques raises the statistical occurrence of contaminating events during the manufacturing process. The elimination of process steps in itself typically results in yield improvement.
The dominant trend in the art is to attempt to increase the accuracy of each step of the photolithography process. Accuracy in etching may be increased by reducing the thickness of the deposited film. However, this increases the requirements for uniformity in the deposition phase. In addition, these thin deposition layers are, in some cases, thin enough to experience and exhibit quantum phenomenon and behavior. However, for the most part, these layers have bulk material properties. This can lead to many uncontrolled parameters during the fabrication stage. For example, a convex surface profile of the deposited layer may lead to strained and mismatched lattices, thus creating devices susceptible to failure through numerous mechanisms.
The pursuit of better photolithography through the application of higher frequency light waves, such as hard ultraviolet and x-rays, is based upon a relationship expressed in the Rayleigh Criterion between the wavelength of a light wave and its corresponding achievable diffraction limit.
The Rayleigh Criterion is given by: EQU d=0.61 .lambda. N.A.
where
.lambda.=operational wavelength of the emitted light; PA1 N.A.=Numerical Aperture of the imaging optic; and PA1 d=diameter of the minimal achievable spot size. PA1 h=Planck's constant=6.62.times.10.sup.-34 J/second; PA1 m=the mass of the atom in Kg; and PA1 v=the velocity of the atom in meters/second.
Replacing .lambda. in the equation of the Rayleigh Criterion with the wavelength .lambda. of a particular light wave will yield the diffraction limit of the light wave emitted at that frequency. Higher frequencies obviously exhibit lower diffraction limits.
The employment of shorter illumination wavelengths does theoretically allow for the definition of significantly smaller device feature sizes. Unfortunately, however, the greater photon energies of higher frequency light waves often create other obstacles to the process of manufacturing smaller geometric features. Higher frequency light waves are more likely to damage lithographic mask optics very quickly because of their high levels of material interaction reactivity. Light waves of wavelengths below 200 nm are more prone to uncontrolled scatter and absorption by the substrate and other deposited materials. Higher energy photons are also more prone to damage the material that they are bombarding.
Conventional photolithography techniques, including those employing x-rays, still typically include an etching step and require the provision of a substrate composed of a pure single crystal, such as silicon or gallium arsenide. Device structures are currently fabricated with atoms in higher energy states, which result in an increased incidence of lattice mismatch and attendant structural fragility.
Furthermore, most conventional photolithography applications entail the production of environmentally hazardous solutions, reagents and by-products. Manufacturing techniques that reduce the generation of biohazardous material by the nano-device industry, and especially the semiconductor device industry, are of significant benefit to the both manufacturers and the world community.
Creative and meaningful work has been done in the area of controlling the deposition of atoms through the Lorenz force effect created by the interactions between an atomic dipole and a standing light wave. Much of this work exploits the generation of a Lorenz force caused by the interaction of an atom, behaving like an oscillating dipole, with the oscillating electric field of a laser. The atom behaves like an oscillating dipole acted on by a Lorenz force, where the Lorenz force is proportional to the intensity gradient of the oscillating electric field of the laser.
J. J. McClelland and M. R. Scheinfein, for a first example, proposed the use of a laser beam as a means of focusing an atomic beam to create nanometer, or nm, scale spots. (J. J. McClelland and M. R. Scheinfein, "Laser focusing of atoms: a particle-optics approach", J. Opt. Soc. Am. B/Vol. 8, No. 9/September 1991, pp. 1974-1986, which is hereby incorporated by reference) McClelland and Scheinfein envisioned the employment of a TEM.sub.01 laser as an atomic lens whereby the direction of an atomic beam is purposely directed with a focal spot size on the order of one nanometer.
T. Sleater et al. have reported on the successful implementation of an atomic lens scheme wherein a cylindrical lens potential was created by positioning a large period, 45 micron, standing light wave perpendicular to a supersonic beam of metastable helium atoms. (T. Sleator, T. Pfau, V. Balykin, and J. Mlynek, "Imaging and Focusing of an Atomic Beam with a Large Period Standing Light Wave", Applied Physics B, 1992, pp. 375-379, which is hereby incorporated by reference) The thinness of the established lens was estimated to be 40 microns. An achieved spot size of four microns was primarily limited by diffraction. Additionally, a microfabricated grating with a period of eight microns was constructed. Chromatic, spherical and diffusive aberrations appeared to have little impact on the spot size. Sleator et al. further suggested that the thinness of the lens could enable lithographic applications in the nanometer range.
Sleator et al., with helium atoms excited by copropagation of electrons at an energy level of about 31 eV, has a resulting beam of metastable helium atoms having an average velocity of approximately 1760 m/s and a corresponding deBroglie wavelength of 0.56 angstroms.
The deBroglie wavelength is calculated from the following expression: EQU l.sub.db =h/mv=
where
Sleator et al. further proposed that their technique could be generalized to two dimensions by combining two standing waves. The creation of two dimensional device structures should thereby be executable. Sleator et al. also predicted that an atomic beam exhibiting a particle velocity of 900 m/s should be focusable with a laser power of ten mW into spot sizes of ten nm.
In another example where neutral atom lithography was accomplished, Timp et al. used an optical standing wave of 589 nm as an array of cylindrical lenses to focus a perpendicular sodium beam and to thereby construct a grating on a substrate, where a periodicity of 294.3+/-0.3 nm was achieved. (G. Timp, R. E. Behringer, D. M. Tennant, and J. E. Cunningham, "Using Light as a Lens for Submicron, Neutral-Atom Lithography", Physical Review Letters, Vol. 69, No. 11, Sep. 14, 1992, pp 1636-1639, which is hereby incorporated by reference.)
G. Timp et al. further described a collimated sodium atomic beam propagating along a y axis and interacting with a perpendicularly oriented standing wave (SW). In preparing the atomic beam for interaction with the standing wave the atomic beam was passed through a "Doppler" optical molasses in order to reduce the transverse velocity and cool the sodium atoms. The average force, U.sub.(z), exerted by the standing wave on the sodium atoms then acts as an array of weak cylindrical lenses and focuses the atomic beam into a grating on the substrate with a period one half of the wavelength of the standing wave, or approximately 589 nm/2=294.5 nm.
J. J. McClelland et al. report the use of laser light to control the motion of a chromium atomic beam to fabricate a nanostructure. (J. J. McClelland, R. E. Scholten, E. C. Palm, and R. J. Celotta "Laser-Focused Atomic Deposition" Science, Vol. 262, Nov. 5, 1993, pp. 877-880, which is hereby incorporated by reference.) The resulting nanostructure consisted of a series of lines which showed line widths of 65 nm+/-6 nm, line spacings of 212.78 nm and heights of 34 nm+/-10 nm.
The techniques of Timp et al., Sleator et al. and McClelland et al., rely upon the application of the two interaction mechanisms existing between laser fields and atoms, namely the spontaneous force and the dipole or gradient force. The dipole force is discussed above as the Lorenz force. The spontaneous force is used in the construction of an optical molasses, where a laser field repeatedly bombards an atom with photons. The atom will then radiate photons in random directions through spontaneous emission. The resultant effect of this atomic absorption and radiation of photons includes a net transfer of momentum to most of the subject atoms in the direction of the absorbed photons, as the momenta of spontaneously emitted photons will statistically average to zero. Thus, the spontaneous force can be used in an optical molasses apparatus to cool an atomic beam.
The conventional art thus shows an interest in using lasers to direct the deposition of atomic beams onto substrates and to form simple structures and features of nano-devices and semiconductor devices. The prior art, however, lacks significant enablement work to allow for the efficient construction of most conventional semiconductor devices, as the creation of large numbers of nanostructures by the direct deposition of atomic beams onto substrates via the application of Lorenz force interactions between atomic beams and light fields requires the production of numerous specific patterns of photonic energy.
The technologies of holographic generation, however, offer efficiencies in the creation of the necessary photonic energy patterns. Commercially available software packages, e.g. the ZEMAX-EE product from Focus Software or Wolfram Research's Mathematica package, are capable of calculating from a desired holographic shape and orientation to define the required diffraction pattern to generate the intended hologram. These mathematically powerful software programs indicate the feasibility of back calculating diffraction patterns upon the basis of a mathematical definition of the desired image.
The design of photonic lenses to focus a beam of atoms using Lorenz force interactions requires that the shape, energy states of individual atoms and isotopic composition of the atomic beam be precisely anticipated. High intensity evanescent waves have been demonstrated by R. Kaiser et al., (R. Kaiser, Y. Levy, N. Vansteenkiste, A. Aspect, W. Seifert, D. Leipold and J. Mlynek "Resonant Enhancement of Evanescent Waves with a Thin Dielectric Waveguide", Optics Communications, Vol. 104, No. 4, 5, 6, (1994), pp. 234-240, which is hereby incorporated by reference.) in conjunction with thin dielectric plates to be used as an atomic mirror. The technique of Kaiser et al. couples a laser beam to a dielectric wave guide by optical tunneling through a solid gap. An enhanced evanescent wave is thereby produced in a vacuum above the wave guide. This evanescent wave functions as an atomic mirror.
There is a long felt need in the industries of nano-device and semiconductor device manufacturing to efficiently and accurately define more robust structural components of devices with smaller dimensions than the prior art allows. There is also a long felt need to limit the environmental impact of manufacturing semiconductors by reducing the volume of toxic chemicals generated for and by semiconductor processing. Alternatives to subtractive processing techniques, whereby material is first applied to a substrate or structure and then selectively removed, may therefore offer significant value to the art of nano-device and semiconductor device manufacture.